In digging and grouting the ground by using a nozzle for injecting a high pressure liquid (hereinafter, referred to as a nozzle) of a consolidating agent injecting apparatus (hereinafter, referred to as a monitor), the shape of the fluid (i.e., jet stream) existing at a position ahead of the nozzle (i.e., the shape of the jet stream injected from the nozzle) is largely influenced by conditions of flow in the pipe prior to the nozzle. Ideally, the flow in the pipe is a laminar flow. However, it is experimentally known that if the velocity of the fluid in the pipe is set to 10 m/sec or less, the influence of these flow conditions against the jet stream can be ignored.
FIG. 3 shows a conventional nozzle 2. Since nozzle 2 is formed in a position arranged in the direction of 90.degree. from a fluid transport pipe passageway 1 (hereinafter, referred to as a pipe passageway), the flowing direction of the fluid is perpendicularly changed with a small radius r of curvature in a short distance l from the pipe passageway 1 to the nozzle 2. Thus, a turning flow occurs in the curved portion of the pipe passageway 1, the vector component in the horizontal direction decreases, and a loss of motive power occurs. In addition, a flow state of the fluid becomes turbulent at a position before the nozzle 2 and the flow passes as turbulent flow in the nozzle 2. As a result, an ideal jet stream of laminar flow cannot be obtained. Furthermore, the value of the diameter of the pipe passageway 1 is small and the flow rate is also small (e.g., 100 liters per minute) so that grouting efficiency is low.
In recent years, a demand has arisen to construct an underground columnar consolidation body of a large section area having a large diameter. However, when the diameter of the pipe passageway is increased and the flow rate is set to a large value (e.g., 300 to 400 liters/min.), the above-mentioned loss of motive power, turbulent flow, and other technical problems will occur.
As is well known, to eliminate the turning flow, assuming that the diameter of the pipe passageway is set to d, a distance of=100 to 150 d is needed. However, if the diameter d is set to a large value such as to eliminate the turning flow, the distance l also increases. In the monitor, the nozzles are mounted at opposite positions to counterbalance the injection reaction forces thereof and the outer diameter of the monitor is generally relatively small (e.g., generally 10 cm). Therefore, if the length of the rectilinear portion of the nozzle is set to be long so as not to cause any turning flow even in the case of a large flow rate, the rectilinear portion cannot be enclosed in the monitor.
On the other hand, the above-mentioned ground improving injecting apparatus is attached to the tip of the pipe. In the case of forming a cylindrical ground improving portion, the pipe is drawn up while being rotated. In the case of forming a vertical flat plate shaped ground improving portion, the pipe is drawn up without rotation thereof. In these manners, the ground improving injecting apparatus is used. However, there are problems similar to those in the foregoing consolidating agent injecting apparatus.
That is, as shown in FIG. 8, the ground improving injecting apparatus ordinarily has one nozzle for injecting the high pressure liquid. In order to reduce the outer diameter of an injecting apparatus 101, a pipe passageway 103 extending to a nozzle 102 is perpendicularly bent in the inlet portion of the nozzle 102. As a result, the jet fluid becomes a turbulent flowing state in a rectilinear portion 104 and directly passes through the nozzle 102 while remaining in the turbulent flowing state. Furthermore, a jet stream J does not achieve a theoretical flow and the grouting capability is low. Thus, in the conventional ground improving injecting apparatus, in order to improve the grouting capability, the injecting pressure and/or flow rate are controlled so as to increase within a fine range, and/or the grouting time is prolonged.
In order to reduce the turbulence of the jet stream J, it is necessary to produce laminar flow so as not to cause any turbulent flow in the rectilinear portion 104. Referring to FIG. 9, such a requirement depends on the pipe diameter d, the length l of the rectilinear portion 104, and the flow rate in the portion 104. As is well known, in order to completely produce laminar flow, the optimum value of l/d should be set to 100.about.150.
On the other hand, a limitation exists in the case of the pipe diameter of double pipes (or triple pipes where three kinds of liquids, i.e., a high pressure liquid, a low pressure liquid, and a ground improving injection liquid, are injected) arranged in the pipe passageway. These pipes transport both the high pressure liquid and the ground improving liquid in the ground improving injecting apparatus. Therefore, in order to realize the optimum value to l/d, the pipe diameter d in the rectilinear portion 104 should be minimized as possible. The desirable upper limit value of the flow velocity for realizing the laminar flow is set to 10 m/sec; however, a flow velocity higher than 10 m/sec is not preferable. That is, since the pipe diameter d is small and there is also a limitation of the flow velocity, the flow rate inevitably decreases. However, as the flow rate decreases, the flying distance of the jet stream J will be relatively short. Thus, grouting capability will deteriorate.
To form an underground columnar consolidation body of a large section area having a large diameter, a large grouting capability is necessary. According to studies by the inventors of the present invention, grouting capability is largely influenced by a discharge amount of the jet stream J rather than a discharge pressure thereof, and flow rate of 300 liters per minute or more is preferable.
In the conventional consolidating agent injecting apparatus and ground improving injecting apparatus, to prevent the jet stream from reaching a turbulent flow state, predetermined limits for the diameter and/or length of the rectilinear portion of the nozzle are necessary. However, as discussed above, various problems arise due to such limitation.
Conventional techniques have been proposed in U.S. Pat. No. 4,084,648, entitled "PROCESS FOR THE HIGH-PRESSURE GROUTING WITHIN THE EARTH AND APPARATUS ADAPTED FOR CARRYING OUT SAME", and U.S. Pat. No. 4,047,580 entitled "HIGH-VELOCITY JET DIGGING METHOD".